Absorbent elements for disposable absorbent articles are commonly formed by different individual material layers which are superimposed, wherein each material layer is designed to provide specific properties. A typical structure for an absorbent element includes an acquisition layer and a storage layer, other layers can also be present such as a distribution layer, a tissue layer, a layer to provide resiliency to the products (bunching resistance), or a layer to provide a better visual impression etc. as known in the art.
The acquisition layer is typically placed on top of the body facing surface of the storage layer and has the function of rapidly acquire the fluids excreted from the body and to transfer them rapidly away from the body into the storage layer, and also to keep the storage layer separate enough from the skin so to avoid that body fluids can rewet the skin during the usage of the absorbent articles. In some cases the acquisition layer has also, as secondary function, the function of distributing the fluid on a larger surface area so to provide a more efficient usage of the surface of the storage layer. In other cases this distribution function can be performed by a portion of the storage layer or by a separate layer having this specific function. This distribution layer can be placed for example below the storage layer on the surface of the storage layer which is opposite to the body facing one (garment facing surface).
In some technical documents typically relating to Feminine care articles the acquisition layer is also called “secondary topsheet”. In the present application, the term “acquisition layer” is intended to be equivalent and to include also “secondary topsheets”.
Each of the mentioned layers can be formed by one or more sub layers, for example the storage layers can be formed by 2 or more sub layers having the same or different functions and/or chemical composition. Also the acquisition layer can independently be formed by more sub layers, having different functions and/or chemical composition. For example, in case the acquisition layer also performs a fluid distribution function, the portion of the acquisition layer closer to the body facing surface of the absorbent element can act transferring the fluids away from the body and the portion closer to the bottom layer can act distributing the fluid along a broader surface before migrating into the storage layer.
A problem associated with using these multilayer structures as absorbent elements in absorbent articles is that fluid transfer from one layer to the other can be non optimal when the layers are separate due to the discontinuity in fluid communication. Traditionally this has been solved by using adhesives such as latexes or hot melt glues at the interface of the layers to bond the layers together, however these adhesive materials can in turn impede the fluid transfer.
In order to solve this problem “unitary” absorbent elements have been developed. The word “unitary” refers to a single structure, which, despite potential internal variations of physical and/or chemical characteristics, is provided such that it cannot be separated into individual layers without destroying the structure or damaging the layers at their interface. Absorbent structures made from a number of layers, which are joined to each other by macroscopic mechanical or adhesive means are not considered unitary since they are formed from individual layers that, albeit sometimes with difficulty, can be separated from each other again.
In other words, similarly to conventional multilayer structures, “unitary” absorbent elements are formed by several layers which can have distinct properties and/or compositions from one to the other. But, while in a “non unitary” absorbent element there is a definite boundary from one layer to the other, in a “unitary” absorbent element the various layers are somehow intermixed at the boundary region so that, instead of a definite boundary between layer it will be possible to identify a region where the different layers transition one into the other. This unitary structure is built forming the various sub-layers one on top of the other in a continuous manner, for example using air laid or wet laid deposition. Typically there is no adhesive used between the sub-layers of the of a unitary material, as this is not necessary due to the unitary construction and the combining being conducted on the layers, however in some cases adhesives and/binders can be present although typically in a lower amount than in multilayer materials formed by separate layers.
Unitary absorbent elements have been disclosed previously e.g. in WO03/090656A1 from Procter & Gamble, US2002/007169A1 from Weyerhaeuser and WO00/74620A1 from Buckeye.
In unitary absorbent elements the fluid communication between the layers is improved, but the performance of these absorbent elements can still be further improved, especially as concerns the performance of the acquisition layer and the fluid transfer properties at the interface between acquisition and storage layer.
The “unitary” absorbent elements described in the prior art are essentially of two types. A first type does not include the fluid acquisition layer in the unitary structure. When these absorbent elements are used in a disposable absorbent article an additional acquisition layer, which is typically required, needs to be applied as a separate layer as in conventional absorbent structures. As a consequence the fluid communication between the acquisition layer and the storage layer suffers of the same drawbacks mentioned above for the non unitary structures.
A second type of unitary absorbent elements described in the prior art includes an integrated acquisition layer which is formed by the same process and with the same technique of the remaining part of the absorbent element.
For example US2002/007169 from Weyerhauser describes unitary absorbent elements produced using a wet laid process, where the various layers are formed one on top of the other, and where also the acquisition layer is formed with a wet laid process.
In WO00/74620A1 from BKI absorbent elements are described which are formed using an air laid process wherein three different layers are deposited in sequence on a tissue carrier and wherein the last layer to be deposited on top is a synthetic PET fibers air laid layer which is supposed to work as an acquisition layer.
These absorbent elements of the prior art can still be improved because the technologies (such as wet or air laid process) which allow the production of desirable storage layer materials, are not always suitable to manufacture acquisition layers having the desired properties.
In particular air and wet laid technologies have inherent limitations in the length of the fibers which can be deposited. In particular the fibers effectively usable in air/wet laid processed are in relatively short, in the range of 8-20 mm. Moreover in materials resulting from air and wet laid methods the fibers tend to be all oriented in the x,y plane, and due to the nature of the air and wet laying methods wherein all the various layers are deposited one on top of the other before further processing of the material, the fibers for the acquisition layer cannot be rearranged or processed independently.
In some cases it is desirable to produce acquisition layers with longer fibers (from 26 to 200 mm long). As known to the skilled person, long fibers can impart peculiar properties to nonwovens especially as concerns fluid handling. Long fibers can generate longer connected pores leading to increased wicking capability as it can be measured using vertical wicking height tests.
Additionally long fibers in a separately made material can be rearranged along the z axis if desired (using consolidation techniques such as hydroentangling or needlepunching) thus allowing to obtain a wide range of finely tunable fluid handling properties. For examples fibers oriented in all direction in a 3D structure can provide tunable resilience and porosity properties to the acquisition layer. Z-direction fibres an also create fluid handling channels that quickly bring the fluid away from the surface.
There is therefore a need to use long fibers in acquisition layers. This is currently possible only by using separate nonwoven layers for example hydroentangled, air trough bonded, needlepunched, spunbond, carded resin bonded and meltblown nonwovens. However the use of a separate nonwoven layer faces the problems (mentioned above) concerning that fluid transfer from one layer to the other is non optimal due to the discontinuity in fluid communication.
The absorbent elements of the present invention have the advantage of using a selected non woven material which can be manufactured separately as acquisition layer. This non woven material can therefore be tailored and tuned to provide the desired fluid transfer properties by forming it and consolidating it using conventional techniques such as e.g. carding, needlepunching or hydroentangling. At the same time an improved integration between acquisition layer and storage layer is achieved due to fiber interpenetration (described in detail below). This allows using less adhesives and binders such as latexes at the interface between acquisition and storage layer (or even no adhesives and latex binders) with the result of maximising the fluid communications between the layers and without compromising on the acquisition properties of the layer.